Well, we don’t know how good the McLaren chassis is, because the engine has held back the testing times.
We do know that McLaren (like Ferrari) made a poor decision on the aerodynamics last year that resulted in excessive understeer. We also know that the appointment of Peter Prodromou has been key to McLaren and that his influence on the MP4/30 has been profound, given the obvious Red Bull influences to the aero package. I’d be willing to bet that the McLaren would be near the front if it had a Merc engine (assuming it would fit under the Honda-specific bodywork).
2014 McLaren & Ferrari analysis here
However, we do know that the Honda hasn’t been given the full moo on track yet and that there’s a lot of work to do. We also know that there have been niggling reliability issues (oil seals…seriously?) with testing the McLaren-Honda.
Frankly, it’ll be a miracle for either Macca to make it to the end of the race. However, I’d be willing to bet that McLaren-Honda are strong in Monaco and Hungary – assuming they get the reliability issues solved by then.